05_06_配置OSPF作为PE和CE路由器之间的路由协议

2019年7月27日 06:59 作者：金淼淋分类：MPLS  817

MPLS VPN Implementation
YES LAB
Configuring OSPF as the Routing
Protocol Between PE and CE Routers
YES LAB
wwwveslabnet OSPF Hierarchical Model
YES LAB
OSPF Area O (Backbone Area)
Area Border Router
Area 1
Area Bo
Area 2
r Router
Area 3
— OSPF divides a network into
all of them
areas,
linked through the backbone (Area 0).
OSPF4$ldJ OSPF in an MPLS VPN Routing Mod
BGP Backbone
PE Router
PE
CE Router
Site IGP Site IGP
YES LAB
uter
Site IGP
— Redistribution between IGP and BGP is performed to
propagate customer routes across the MPLS VPN
backbone. OSPF Superbackbone:
OSPF-BGP Hierarchy Issue
YES LAB
BGP Backbone
PE Router
PE
uter
Area O Area 2
Area O Area 3
OSPF Area O might extend into individual sites.
• The MPLS VPN backbone has to become a superbackbone for OSPF. OSPF in MPLSVPNs:
Goals
YES LAB
一不同站点之间的 OSPF 不能采用通常的 OSPF ． BGP 重分布方
法
一在跨越 MPLS VPN 骨干网络的时候， OSPF 的连续性必须得
以保持：
· OSPF 的内部路由保持是内部路由
· OSPF 的外部路由保持是外部路由
· OSPF metrics 需要保留
一 CE 路由器运行标准的 OSPF ．
wwwvesla.com/n wwwveslabnet OSPF Superbackbone:
Rules
•OSPF superbackbone behaves exactly like Area 0 in
regular OSPF:
wwwveslabnet
YES LAB OSPF Superbackbone:
Implementation
— Extended BGP communities are used to propagate OSPF
route type across BGP backbone.
— OSPF cost is copied into MED attribute. OSPFJji2tOÉZ
fblJ
wwwveslabnet
YES LAB OSPF Superbackbone:
External Routes
BGP
10.0.0.0/8
Backbone
OSPF RT=I
MED=20
YES LAB
PE Router
10.0.0.0/8
LSA type 5
E2 metric 20
Area 1
PE Router
10.0.0.0/8
LSA type 5
E2 metric 20
Area 2
External OSPF routes are propagated in the same way as internal OSPF routes across
the superbackbone.91• 0
External metric and route type are preserved. OSPF Superbackbone:
Mixing Routing Protocols
YES LAB
BGP
Backbone
PE Router
10.0.0.0/8
Hop count 3
10.0.0.0/8
MED-3
PE Router
10.0.0.0/8
LSA type 5
E2 metric 20
Area 2
Routes from the MP-BGP backbone that did not originate in OSPF are still
subject to standard redistribution behavior when inserted into
OSPFL+
wuwveslabnet Configuring PE-CE OSPF Rou
YES LAB
•Follow these steps to configure OSPF as the PE-
CE routing protocol:
— Configure per-VRF copy of OSPF.
— Configure redistribution of MP-BGP into OSPF.
WJMd±lJOSPF0
— Configure redistribution of OSPF into MP-BGP.
fidHOSPFWJ}fid
ÉIJMP-BGPO
wwwveslabnet OSPF Down Bit:
Loop Prevention
YES LAB
— An additional bit (down bit) has been introduced in the
options field of the OSPF LSA header.fiospF
GIA r o
— PE routers set the down bit when redistributing routes from
MP-BGP into OSPF.
downffro
— PE routers never redistribute OSPF routes with the down bit
set into MP-BGP.
wwwveslabnet Optimizing of Packet Forwarding
Across the MPLS VPN Backbone (Co
1. OS fL
1. The OSPF route is propagated
with the down bit set.
BGP Backbone
YES LAB
2. The OSPF route is ignored
because the down bit is set.
PE Router
Area I
PE Router
PE
Down
Area 2
uter
Another Site
(Packet flow across the network is optimal.)
wwwveslabnet PE Router
Site 1
Sham Link
High-Bandwidth
BGP Backbone
PE Router
Low-Bandwidth
Backdoor Link
Area I Site 2
YES LAB
— OSPF pre ers Intra-area pat s to Interarea pat s.
— The path overa backdoor link will always be selected.
wwwveslabnet Sham Link (Cont.)
YES LAB
— A logical intra-area link.
— Carried by the superbackbone.
— A sham link is required only between two VPN sites that belong to
the same area and have a backdoor link for backup purposes.f1ZEJ/ii
— OSPF adjacency is established acrossthe sham link.
— LSA flooding occurs across the sham link.
wwwveslabnet Sham Link (Cont.)
YES LAB
•When a sham-link route is preferred by OSPF:
YOSPF
— The OSPF route is not redistributed to MP-BGP. OSPFff6ffi
8WhfidåMP-BGP0
— Instead, the router on the other end of the sham link
performs the redistribution.
— The forwarding information from the MP-BGP route is
used.
wwwveslabnet Sham Link (Cont•)
High-Bandwidth
BGP Backbone
Packet flow is now optimized.
Sham Link
PE Router
PE Router
Site 1
Area 1
Low-Bandwidth
Backdoor Link
Site 2 Configuring a Sham Link
YES LAB
— A separate /32 address space is required in each PE router
for each sham link.
32
— This/32
• Is required so that OSPF packets can be sent over the
VPN backbone to the remote end of the sham linko
• Must belongtotheVRF0
• Must not be advertised byOSPF0
• Must be advertised by BGP0
wwwveslabnet